The invention is directed towards the field of analog circuitry, particularly filters that manage the transition from differential to single-ended circuits.
The prior art uses two schemes for high frequency analog electronics: single ended and differential. In single-ended designs, the signal is referenced to the ground plane. Any design, e.g. mixing, amplifying, or signal generation, may be performed by a single transistor. This makes the single ended architecture the better choice for discrete implementation. Unfortunately, single-ended implementations have a great sensitivity to the electrical connection to the ground plane. The sensitivity increases proportional to the increase in frequency or power. Compensating for this sensitivity at high power and frequency can require tens to hundreds of bond wires connecting the transistor to the ground plane. Any inductance in the ground plane itself can cause signal leakage between different parts of the larger circuit.
In differential designs, the signal is referenced to two orthogonal arms of the circuit. Any connection to the ground plane is parasitic, and of much less influence. The circuit is much more complex, however, requiring at least three transistors for each function. Thus, differential circuits are virtually always integrated. Another disadvantage is that some voltage is lost in extra biasing elements. However, the isolating nature of the differential is such a significant advantage that the loss of headroom and increased complexity is tolerable.
There are two types of differential structures: ladder and lattice. These filters are typically built using lumped resonators, e.g. crystal resonators, LC resonators, and combinations of the two. These filters tend to be quite large and expensive, hence, too costly for high volume consumer electronic applications, e.g. cellular phones. Recent developments in resonant structures allow these complex circuits to be built in a cost-effective manner.
Current implementations of film bulk acoustic resonators (FBAR) filters are half-ladder, single-ended structures, as shown in FIGS. 1A-B. The helper inductors, shown in FIG. 1B), move some of the rejection further into the reject bands to allow for broader frequency rejection. These helpers cause the rejection slope at the band edge to flatten, thereby reducing the rejection just out of band. The helpers also result in less rejection far out of band.
FIGS. 2A-F show embodiments of the prior art ladder circuits. FIG. 2A is a completely differential unrefererenced ladder section while FIG. 2B is a ladder section with a ground reference. The ground reference increases the filter complexity but has common mode rejection. They function similarly for differential operation. FIGS. 2C-D illustrate the filter response during differential operation. FIG. 2C shows the close-in or narrow-band filter response while FIG. 2D illustrates the broadband response. The filter has a very steep rejection followed by a very fast flyback response. FIG. 2E illustrates a ladder circuit using xe2x80x9chelperxe2x80x9d inductors. This is at the expense of increased complexity and cost. Similar to a half ladder circuit, the band edge rejection is worse. FIG. 2F shows a ladder section with two half ladder in parallel, helper inductors, and a ground reference. This is twice the expense. In each case, there is a very sharp transition from passband to reject band.
FIGS. 3A-3C illustrate the structure and the frequency response for a prior art lattice circuit. FA and Fb refer to different resonant frequencies. Far out of band, the lattice has excellent rejection. However, the near band rejection is very slow. This makes the lattice unusable for circuits requiring close in rejection. This circuit has no fly back at all. Once beyond the passband, the rejection simply increases, at least to the level where other circuit parasitics dominate.
In a first embodiment, an N-stage ladder circuit is serially connected to a bridge lattice circuit. The N-stage ladder circuit receives differential inputs, e.g. mixers, while the bridge lattice circuit outputs a singled end output, e.g. amplifier. In a second embodiment, a bridge lattice circuit is serially connected to an N-stage ladder circuit. The bridge lattice circuit receives a differential source while the N-stage ladder circuit drives a differential output load.
In either embodiment, it is preferable that a film bulk acoustic resonator (FBAR) be used in the combination ladder and lattice structure. This provides the necessary band-pass functionality and makes the transition from differential to single ended load where necessary.